The goal of this revised proposal is to evaluate key mechanisms that we hypothesize underlie the susceptibility of the premature brain to hypoxic-ischemic injury, including neuronal and glial maturation, cerebrovascular responses, NMDA mediated excitotoxicity, the pattern of the insult, previous insults and vulnerability to oxygen free radicals. Systemic asphyxia will be induced in premature fetal sheep in utero by reversible umbilical cord occlusion (Specific aims 1-4), or carotid occlusion, to study the neural response independently of systemic factors (Specific aim 5). The fetal electroencephalogram (EEG), cortical impedance, carotid blood flow and cerebral blood volume will be continuously monitored, regional cerebral blood flow will be determined at critical time points, and subsequent neuronal and glial cell loss quantified. Specific aim 1 tests the hypothesis that there are maturational changes in regional vulnerability of neurons and glia and risk of intraventricular hemorrhage, which are correlated with the cerebrovascular responses to asphyxia and post asphyxial seizures. Studies will be performed at two ages, before the onset of cortical myelination (0.6 gestation (ga)) and during early myelination (0.7 ga). Specific aim 2 will test the hypotheses that post-asphyxial NMDA receptor mediated excitotoxicity contributes to neuronal loss at 0.6 and 0.7 ga, and that fluctuations in fetal blood pressure due to post asphyxial NMDA mediated seizures at 0.7 ga are associated with intraventricular hemorrhage (IVH). Specific aim 3 will test the hypotheses that repeated asphyxial insults induce cortical sensitization, and that the development of cortical injury is related to the development of a post-asphyxial mismatch between cortical blood flow, cortical EEG activity, and global cerebral metabolism. Specific aim 4 will evaluate the hypothesis that a mid-gestational asphyxial injury alters subsequent neural and cardiovascular development, and impairs the ability of the fetus to mount appropriate cardiovascular and cerebrovascular responses to a further asphyxial insult in late gestation. Specific aim 5 will address the hypothesis that there are maturational changes in hydroxyl radical activity and lipid peroxidation that are associated with the development of periventricular white matter lesions, and whether pharmacological protection of plasma membrane integrity reduces glial cell loss in the preterm fetus. By further elucidating the pathogenesis of prenatal and perinatal asphyxial encephalopathy these studies will provide important information towards improving perinatal diagnosis, monitoring and intervention.